Finely powdered products, such as coal fines, ore and other mineral fines, pharmaceutical powders, cosmetic powders and the like, are often dried using fluidized-bed heat exchangers with hot air being used as the fluidizing medium. In addition to fluidizing the powder bed, the hot air heats the powder to drive off the moisture contained therein and acts as a vehicle to carry off the vaporized moisture. The exhaust air for such heat exchangers usually is highly contaminated with fine powder carry-over which must be run through a cyclone separator to recover the carry-over and also an exhaust scrubber to reduce the resulting air pollution to an acceptable level. The equipment required for such a dryer is, at a minimum, a blower, an air heater, a fluidizing vessel, a product conveying system, a cyclone separator, an exhaust air scrubber, and the connected piping. From the product standpoint, this dryer produces highly stratified, loose dusty powder which generally is undesirable. From an energy standpoint, such systems are quite wasteful since the air must be heated to a sufficiently high temperature to vaporize the moisture. The heated air elevates the temperature of the above mentioned equipment thereby contributing to significant heat loss in addition to the loss involved in raising the temperature of the air which is exhausted.
In some operations, such as coal mining, environmental considerations have required the expenditure of much more energy to recover the coal fines than is available from the fines and therefore is inherently wasteful. Also, in many coal mines, the total volume of coal fines which must be dried is so great as to undesirably effect the economic operation of the entire mine.
Microwave heated fluidized-bed dryers such as shown in U.S. Pat. No. 3,528,179 to F. J. Smith are known wherein the operation of a fluidized-bed dryer is improved through the application of microwave energy. However, such does not remove the limitations of fluidized-bed dryers which require complex separation equipment at their outlets to separate the fluidizing medium from the dried product as described above. Microwave energy also has been used to dry food products such as is shown by Jeppson in U.S. Pat. No. 3,409,447 whereas vacuum dryers are described by J. R. Richardson and R. Elwess in U.S. Pat. Nos. 1,920,107 and 2,799,947 respectively. All of the above patents were cited in U.S. Pat. No. 4,015,341 referenced above whose disclosed apparatus is primarily concerned with drying seed in a manner which improves its percentage of germination when compared to previously used methods. The device of U.S. Pat. No. 4,015,341 introduces microwave energy and seeds into a low pressure environment so that the seeds are dried at essentially ambient temperatures and with greatly reduced energy requirements when compared to other electrical dryers.